Wheelbase question for my cub scouts’ cars

[Prozach2]

What wheel base would you use?

We’re doing ladder bodies without fenders. I have a picture below of our first attempt.

I plan on recutting it w/ a shorter wheelbase though. That wheel base is 5 3/8”. Honestly, not sure where I came up with that. It might be from the 5kidsracing instructions. It looks like 4 3/4” is standard with some people on here preferring 5”. Is the 4 3/4” the recommended wheelbase even if I’m not using fenders? What’s the advantage/disadvantage of 5” vs 4 3/4”?

Not sure if any of this matters but, here’s a summary of what we’re doing: we’re drilling axle holes with the silver bullet extreme. 3 degree for rear wheels, zero degrees in front with 3 degree bend in the FDW’s axle. Other front wheel will be raised. We plan to add a rib reinforced with birch or balsa. Does it matter which? We have both. We’ll do my best to get the COG and steer right once I’ve made up my mind on the wheelbase.

Any advice, suggestions, etc. are greatly appreciated!

 

[TRE]

Most pros run 4 and 3/4

 

[Prozach2]

Most pros run 4 and 3/4

I've heard that people started shrinking the wheel bases to make room for the fenders. If you weren't using the fenders, do you still think the 4 and 3/4 is optimal?

 

[TRE]

Yep

 

[Prozach2]

Great. Thank you!

 

[Prozach2]

Here’s the new one with a 4 and 3/4” wheel base. I also ordered some fenders from DD4H since my kid really wants to do fenders.

Please let me know if y’all see any room for improvement!

 

[Brian Stanley]

Looks good!

 

[Prozach2]

Looks good!

Thanks. I really hope this one works. I've managed to screw up 5 blocks, and this is my last block. I'm like the Thomas Edison of pinewood derby cars; I've succeeded in finding 10,000 ways not to build a pinewood derby car!

Of course, it's no big deal to buy more blocks. We love building them. Let me know if you see anything I can improve on.

 

[Mojo Racing]

I like the looks of the bodies...

just curious... what happened to the others bodies?

 

[Brian Stanley]

Thanks. I really hope this one works. I've managed to screw up 5 blocks, and this is my last block. I'm like the Thomas Edison of pinewood derby cars; I've succeeded in finding 10,000 ways not to build a pinewood derby car!

Of course, it's no big deal to buy more blocks. We love building them. Let me know if you see anything I can improve on.

Zach, if you mess this one up let me know, I've got plenty of blanks cut from the BSA blocks, I'll run one off on my router for you if you want.

 

[Prozach2]

I like the looks of the bodies...

just curious... what happened to the others bodies?

I tried to drill holes so I could see the ends of the axles. The drill bit took a chunk out. In hind sight, I think I should’ve drilled those before I hollowed it out with the scroll saw. Correct me if aim wrong but I didn’t think it was worth the risk to drill those again.

Took me a couple tries to get a nose I liked. The nose looked uneven or too thin. Probably would break.

Put the FDW on the wrong side.

Scrapped a couple b/c I built them with the larger wheel base.

 

[Prozach2]

Zach, if you mess this one up let me know, I've got plenty of blanks cut from the BSA blocks, I'll run one off on my router for you if you want.

I really appreciate it. I plan to buy a bunch of blocks, but if there’s a shortage, I’ll definitely take you up on that!

 

[CastleCrasher]

I had basically the same question as Prozach2's original post. This time last year in my totality of research I gathered that an extended wheelbase was preferred, but this year it's not. We're in cub scout pinewood derby, not pro league. So last year we ran a 5 5/16" wheelbase, won pack race but ran about 17-20th place at district, just below the 15 place semifinals. We're not doing fenders, so now I gather that 4 3/4 is still optimal regardless of using fenders? Where should that wheel base be set on the block? Biased to rear? Last year we had 2 oz at the back, then axle 7/8 from rear of block. COG was 5/8-3/4 in front of rear axles.

 

[Prozach2]

I had basically the same question as Prozach2's original post. This time last year in my totality of research I gathered that an extended wheelbase was preferred, but this year it's not. We're in cub scout pinewood derby, not pro league. So last year we ran a 5 5/16" wheelbase, won pack race but ran about 17-20th place at district, just below the 15 place semifinals. We're not doing fenders, so now I gather that 4 3/4 is still optimal regardless of using fenders? Where should that wheel base be set on the block? Biased to rear? Last year we had 2 oz at the back, then axle 7/8 from rear of block. COG was 5/8-3/4 in front of rear axles.

So yeah, same here. Looking at my cars from this year (4 and 3/4 wheel base) vs my cars from last year (5 and 5/16" wheel base), I'm guessing (emphasis on guessing) that the wheels are getting moved back by a lot of the pros b/c, in addition to the fenders, it moves the COG back. My theory is, if you tune the car perfectly, that's going to be an advantage. More weight further back means more momentum. and speed. If you don't tune the car well, with that shorter wheel base, you might have some pretty serious wobbles though. Again, this is all a guess from someone that's far from an expert.

Last year, I had a much lighter car body, more tungsten cubes, and a COG that was pushed farther back than the year before. We put 10x more time and research into last year's car and expected a much better result. We won the pack race both years, but last years car was actually a little slower than the year before. Watching it go down the track, it's no mystery why. It was bouncing back and forth so hard, I was surprised that it didn't go flying off the track. It was a good track in good condition. Fortunately, it stayed on the track, but the experience was a valuable lesson. It's important to know your limitations. Depending on the skill level of the builder, at some point, less is more.

This year, we still don't have a decent tuning table. I'm just putting the 5kidsracing printout on a 5'table and rolling the cars on that. No matter how tight and flat we try to get that huge piece of paper, we're not getting consistent steer. I'm concerned the paper is still bubbling up just enough to alter the direction of the car. I'm looking for a piece of glass or something to put on top of it, but haven't found anything good yet. Our race is on 2/6, so we've got a little time to get it figured out.

 

[Mojo Racing]

Think of it this way, guys…

With the front wheels farther back, the car stays on the decline of the hill for a longer period of time before the body starts to enter the transition. The longer the car can stay on the hill before it starts to transition to flat, the longer the effect of gravity has of pulling your car down.

Once the car starts into the transition, you lose the full force of the gravitational pull, and once you get onto the flat, you only start slowing down. This is the reason most of the league racers don't race long wheel bases (except in unlimited).


As far as your tuning graphic... I don't have glass on my tuning table... I got a piece of 1/16" lexan sheet from Lowe's for a lot better price, and it works great. I have been using it for 6 years now.

 

[Prozach2]

Think of it this way, guys…

With the front wheels farther back, the car stays on the decline of the hill for a longer period of time before the body starts to enter the transition. The longer the car can stay on the hill before it starts to transition to flat, the longer the effect of gravity has of pulling your car down.

Once the car starts into the transition, you lose the full force of the gravitational pull, and once you get onto the flat, you only start slowing down. This is the reason most of the league racers don't race long wheel bases (except in unlimited).


As far as your tuning graphic... I don't have glass on my tuning table... I got a piece of 1/16" lexan sheet from Lowe's for a lot better price, and it works great. I have been using it for 6 years now.

Thanks. That makes a lot of sense. And thanks for the tip on the lexan sheet. I’ll definitely grab that this weekend. That’ll be a huge help.

 

[Mojo Racing]

A friend of mine brought up a good point to me... If you have a smoother transition like on a wood track or a Freedom track, the shorter wheelbase will not make as much of a difference as it does on a Best Track.

 

[Prozach2]

A friend of mine brought up a good point to me... If you have a smoother transition like on a wood track or a Freedom track, the shorter wheelbase will not make as much of a difference as it does on a Best Track.

Our pack and counsel uses a best track.

 

[CastleCrasher]

Prozach2 I was also going to suggest a sheet of acrylic plexiglass. That's all I use, no fancy graphic, just a tape measure. I could certainly see longer wheelbase being more stable but it also makes sense that if your alignment is right it would be stable on a decent track.

I forgot to note that I accidentally had steering set to 6-7" over 30" at the district race, so I'm fairly sure if not for that we would have made semifinals. I swapped wheels and axles for district and couldn't remember what steer amount to use. Oooops!

Mojo, I get your concept of staying on the hill longer, I guess I only considered the cog being the primary factor. As far as the smooth vs sharp transition, that's curious. It makes sense but at the same time, either would still have the same total fall distance as in elevation change. It would be nice to see 2 equal height and quality tracks but with one smooth and one sharp on the transition and compare race times.

 

[Prozach2]

Prozach2 I was also going to suggest a sheet of acrylic plexiglass. That's all I use, no fancy graphic, just a tape measure. I could certainly see longer wheelbase being more stable but it also makes sense that if your alignment is right it would be stable on a decent track.

I forgot to note that I accidentally had steering set to 6-7" over 30" at the district race, so I'm fairly sure if not for that we would have made semifinals. I swapped wheels and axles for district and couldn't remember what steer amount to use. Oooops!

Mojo, I get your concept of staying on the hill longer, I guess I only considered the cog being the primary factor. As far as the smooth vs sharp transition, that's curious. It makes sense but at the same time, either would still have the same total fall distance as in elevation change. It would be nice to see 2 equal height and quality tracks but with one smooth and one sharp on the transition and compare race times.

I'm curious if all the cars that beat you at districts came from the same workshop. I bet one or two scout leaders in the know helped about 20 kids build their cars. At most of the cub scout PWDs I've been to, there have only been a handful of cars built by someone that had a clue how to build a fast car.